Furnace for testing materials and characterization method using the furnace

ABSTRACT

A furnace comprising both overall heating means and local heating means for generating both an ambient temperature and local heating of the specimen so as to be able to simulate a mechanical shock. It therefore lends itself to various tests capable of providing a better evaluation of the self-heating or self-ignition characteristics of a material subjected to a thermal or mechanical stress.

The subject of the invention is a furnace for testing materials associated with a characterisation method using said furnace.

A thermal test for characterising materials relates to their resistance to self-ignition under the effect of external heating. A specimen of a material may be placed in an enclosure and subjected to a conventional heating, such as a temperature ramp as a function of time or a temperature holding period for a determined time. The measurement may be visual, the specimen being observed until a flame or an ember appears, or consist in a measurement of the temperature of the specimen which indicates the additional temperature attained compared to the temperature applied.

Several embodiments of the test exist. One of them uses a powder specimen laid in a layer on a heating plate subjected to a temperature holding period of 30 minutes. The test is considered as positive when the appearance of an ember, flame or a rise in temperature of at least 250 degrees Celsius is observed in the layer compared to the holding period temperature.

A second test embodiment uses a Goddert-Greenwald furnace which mainly comprises a vertical cylindrical tube heated by a resistance to the desired temperature. The powder specimen is laid in a horizontal tube communicating with the vertical cylinder. A high pressure system blows a cloud of powder into the vertical tube. The test is positive if flames appear.

Finally, a third test consists in placing a specimen in a steel basket which is placed in a furnace. The desired temperature is established in the furnace. The test is considered as positive by convention when the temperature of the specimen exceeds 400 degrees Celsius.

The very variety of these furnaces and the criteria retained to evaluate the self-ignition temperature bears witness to the arbitrariness of this measurement. Above all, it is not certain that the self-ignition should be related to the overall conditions in the material, since it can often proceed from shocks or other mechanical interactions having dissipated the energy in a small volume.

There exists a certain number of furnaces, of which the documents EP A 1,132,733, U.S. Pat. No. 3,987,661 and U.S. Pat. No. 3,718,437 give examples, which describe furnaces provided with overall heating means, which act on the specimen indirectly by heating firstly, and rather uniformly, the gaseous medium surrounding the specimen.

A furnace of a novel kind is proposed according to the invention.

It relates to a furnace for testing materials comprising an enclosure, a tray for receiving specimens of materials, above said tray first adjustable, overall heating means, which are spread out on the periphery of the enclosure, and second heating means, housed in the tray and passing through the surface of the tray so as to be in contact with the specimen, extending up into a place where it is received.

The second heating means have a heating intensity adjustable independently of the first means and have a defined geometry (shape and dimension) in order to apply according to the desired conditions an additional energy directly to the specimen independently of the ambient temperature in the enclosure, controlled by the overall heating means. In this way, more realistic conditions are available to characterise the thermal behaviour of materials with heating and particularly their aptitude to self-ignition, the second heating means being able to have a reduced surface area to simulate local heating, safeguarded by the direct contact between them and the specimen.

Advantageously, the tray forms part of the enclosure, which is also composed of a bell laid on the tray and separable from the tray, the overall heating means are spread out particularly on the tray and around at least one lower part of the bell, and the second heating means extend beyond the upper surface of the tray.

Such an embodiment with bell and tray is particularly simple to manufacture and handle, which makes it useful for the study of radioactive materials through a glove box or any other protective wall. The spreading of the first heating means procures a uniform heating of the specimen, and the second heating means act in the very interior thereof, which may represent a more realistic test.

Greater security is offered if the furnace comprises screws to unite the bell to the tray, and springs assembled against the screws to tolerate a spreading between the bell and the tray in the event of high pressure in the enclosure.

Finally, the invention relates to an original method of measuring the self-ignition temperature of a specimen. It consists in evaluating the effect of a mechanical energy on the tendency to self-ignition of the material by evaluating a quantity of energy dissipated in the material with a mechanical energy, in placing a specimen of the material in the furnace defined above, and in applying the quantity of energy to the specimen by the heating means of defined dimension, before observing or measuring the effect on the specimen.

The invention will be described by means of FIGS. 1 and 2, which correspond to two views of the furnace in oblique view and side view and FIG. 3, which represents more schematically the furnace in section. The furnace is firstly composed of a stainless steel bell 1 with three viewing ports 2 equipped with cooled glass, below which is a tray 3, the vertical movement of which is controlled by an electric motor 4. The tray 3 bears vertical columns 20 on which slide a support 21 of the bell 1, and the motor 4 makes a worm screw 22 turn which raises or lowers the tray 3 by an ordinary kind of transmission. The tray 3 may thus be pressed against the bottom of the bell 1 or separated from it. Between the bell 1 and a protective hood 5 that surrounds it are installed an electrical resistance 7 adjacent to the bell 1 and an insulating layer 8 adjacent to the hood 5. The resistance 7 is a winding strip extending at least to the lower part of the bell 1. The insulating layer 8 extends all around the bell 1, and above it. The hood 5 is cooled by a fluid circuit 6 extending on the peripheral face and summit thereof. The tray 3 comprises an electrical resistance 9 in planar form extending over a part of the surface thereof and, at the centre, a local electric resistance 10. The electrical resistances 7 and 9 together constitute a first, overall heating means, creating a heating throughout the enclosure composed of the tray 3 and the bell 1, and the local electrical resistance 10 constitutes a second heating means that is exerted on a small, localised volume, of the specimen laid on the tray. It may consist in a pointed filament passing through the upper surface of the tray 3, flush with it or extending upwards from it to a place such as a receptacle 22 where the specimen is received by being laid on this upper surface. These heating means are adjustable and independent. Through the tray 3 also traverse thermocouples 11, a conduit for extracting gases 12 equipped with a pressure sensor 13 and a safety valve 14, a pipe for supplying gases 15 equipped with a flow regulator, not represented. The supply pipe 15 is intended to fill the enclosure with the gas desired for the test. During closing, the tray 3 seals the bell 1, and three screws 16, borne by the support 21 of the bell 1 and screwed into the tray 3, make it possible to seal the enclosure in a leak tight manner. Springs 17 are nevertheless assembled against the screws 16 so as to enable the opening of the bell 1 in the event of high pressure of the gases, which compresses said springs 17. The furnace is equipped with devices for acquiring normal data such as recorders of the internal pressure, temperatures of the specimen and the enclosure, as well as an optical pyrometer, a camera and a flow regulator. The device may be completed by gas analysers at the outlet, a generator of humid gas, a thermal camera, etc. All of these devices are centralised on a computer close to the work station and which carry out the desired temperature regulation. Means of lighting the enclosure are provided.

The furnace may be placed in a glove box that receives the fluids necessary for the test, as well as the specimens of materials and auxiliary means such as a balance enabling the specimens to be weighed, and other sensors. The volume of the enclosure may be around 5 litres and the temperature applied may reach up to around 500 degrees Celsius. The safety valve may be tared to 3 bars and the springs 17 for opening the enclosure to 5 bars. Finally, the wall of the enclosure has been dimensioned and tested for a pressure above 10 bars. The pressure sensor may trigger an automatic stoppage of the heating as soon as a pressure such as 1.5 bars is reached.

A test may be undertaken in the following manner. A specimen 23 of material to be tested is placed in the enclosure, for example in the receptacle 22 formed at the centre of the tray 3, and the enclosure is closed by raising the tray 3. The sealing is ensured by a circular seal 24 completed by an adjacent cooling means, which are not represented but may further comprise a liquid coil. The overall heating means composed of electrical resistances 7 and 9 are then started according to the test specifications either by applying a heating ramp (around 5° C./min), or isothermally, to determine an ambient temperature. The electrical resistance 9 on the tray 3, beside the specimen 23, does not heat it directly. When a mechanical interaction dissipating energy in the specimen has to be simulated, its quantity of energy is evaluated by calculation, empirically, or otherwise, and it is delivered by the local electrical resistance 10. This application of heat is much better for simulating a mechanical interaction since it is made in actual contact with the specimen, such as a rubbing or a shock, and since it is exerted on an area of defined dimension, which is often true again for mechanical interactions. The judgement on the result of the test is then obtained by applying a criterion chosen by the user, such as those already proposed: optical examination of the specimen on measuring its rise in temperature.

The second heating means may have another form or another surface area, such that the invention is not limited to a localised heating. It may pass through the upper surface of the tray 3 to extend beyond it to be inside the specimen 23 when it is powdery or divided, or to be flush with this surface to end up in contact with the surface of the specimen, especially when it is solid. 

1. Furnace for testing materials, comprising an enclosure, a tray for receiving specimens of materials at an upper surface, and a first adjustable heating means, which is overall and spread around at least one part of the enclosure, characterised in that it further comprises a second adjustable heating means, which is of defined geometry, passing through the upper surface of the tray and extending up into a place for receiving a specimen.
 2. Furnace for testing materials according to claim 1, characterised in that the tray forms part of the enclosure, which is also composed of a bell laid on the tray and which can be separated from the tray, the first overall heating means are spread out on the tray and around a lower part of the bell, and the second heating means extend beyond the upper surface of the tray.
 3. Furnace for testing materials according to claim 2, characterised in that it comprises screws to assemble the bell on the tray in a sealed manner, and springs assembled against the screws to tolerate a spacing between the bell and the tray in the event of high pressure in the enclosure.
 4. Furnace for testing materials according to claim 1, characterised in that the second adjustable heating means consist in a pointed filament extending beyond the upper surface of the tray.
 5. Method of characterising a material with heating under the effect of a mechanical energy, characterised in that it consists in evaluating a quantity of energy expended in the material by said mechanical energy, placing a specimen of the material in the furnace according to claim 1, and applying said quantity of energy to the specimen by the second heating means.
 6. Method of characterising the thermal behaviour of a material with heating under the effect of a fixed temperature, or a temperature ramp chosen by means of the furnace according to claim
 1. 